Mount Prospect, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

262
FXUS63 KLOT 212324
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
624 PM CDT Sun Jun 21 2026

.KEY MESSAGES...

- Widespread soaking rainfall is expected across the area
  through tonight. A corridor of locally higher rainfall rates
  may occur near/south of I-80 leading to localized flooding.

- There is a low chance (20-30%) for isolated non-severe
  thunderstorms south of a Pontiac to Rensselaer line this late
  afternoon through early evening.

- A heightened risk for rip currents will exist at southern Lake
  Michigan beaches from this evening through Monday night.

&&

.DISCUSSION...
Issued at 211 PM CDT Sun Jun 21 2026

A pair of shortwave troughs continue to phase over the Upper
Midwest with a broad area of showers spreading across eastern
IA, IL, and northwest IN ahead of these features. The earlier
MCV that was traversing central IL seems to have weakened with
now only a broad area of lower pressure being analyzed over
northern MO. Extending from this low is a warm front which
stretches from the IA-MO border southeast across central IL into
central IN. Given the ongoing showers and the dissipating MCV,
the warm front is expected to stay near and south of a
Bloomington to Lafayette line which should keep the better
surface instability south of our area. As a result the threat
for strong to severe storms into northeast IL and northwest IN
has greatly decreased. In fact the Storm Prediction Center has
pulled the marginal (level 1 of 5 risk) from our area for today.
However, there is still a chance (20-30%) for an isolated
embedded thunderstorm or two later this afternoon and evening as
the approaching troughs aid in steepening the mid-level lapse
rates. If any storms occur the main area for them looks to be
along and south of a Pontiac, IL to Rensselaer, IN line.

Regardless of the thunderstorm coverage, the main forecast
concern through this evening will be the threat for locally
heavy rainfall. The reason is because as the aforementioned
surface low approaches a mid-level baroclinic zone is expected
to establish overhead and interact with the broad forcing from
the shortwaves and the modest instability. Couple these forcing
mechanisms with the relatively high moisture content (PWATs
around 1.3 to 1.5 inches) and conditions are prime for periods
of heavier rainfall. Latest trends continue to support that the
greatest potential for heavy rainfall will be near and south of
I-80 closer to the baroclinic zone and slightly better
instability. As a result, total rainfall in south of I-80 may
get into the 1-2 inch range with locally higher totals possible
which may result in pockets of flooding especially in lower-
lying and poor drainage areas. Furthermore, there is also the
chance for an additional axis of heavier rainfall to develop
along the system`s deformation band this evening. While
confidence on the placement of this deformation band is still
low, trends continue to suggest it may set up between the I-88
and I-80 corridors. If this does set up here, the large
urbanized area may allow the 1-1.5 inch rain amounts to result
in more instances of flooding and ponding of lower-lying and
poor drainage areas. Due to the long duration of the rainfall
(12-14 hours) and the modest rain rates (around 0.5 to 0.75
inches per hour at worst), confidence remains too low to warrant
the issuance of a Flood Watch at this time. That said, areas
south of I- 88 remain in the level 1 and 2 of 4 risk areas for
flooding from the Weather Prediction Center.

Heading into tonight, the surface low and the associated
shortwave wave will pivot east across the area which will allow
rain to gradually taper after midnight and through early Monday
morning. Though, depending on how fast the system exits a few
showers may linger across portions of eastern IL and northwest
IN through mid-morning Monday. Otherwise, expect mostly dry
conditions for Monday with skies gradually clearing through the
day. Winds, however, will become breezy behind the surface low
this evening and will remain as such through Monday with gusts
around 20-30 mph. Given the northeast wind directions, these
winds will result in building waves (upwards of 4 to 7 feet) and
dangerous rip currents on Lake Michigan. Therefore, a Beach
Hazard Statement remains in effect for this evening through
Monday night so be sure to stay dry when waves are high.
Temperatures on Monday will also be on the seasonably cool side
with highs in the mid-70s, but cooler conditions (mid to upper
60s) are forecast for areas near the lake due to onshore winds.

Winds will subside Monday night into Tuesday as a surface high
pivots into the Great Lakes. While wind directions look to
maintain a northerly component on Tuesday, temperatures are
expected to be warmer with highs in the upper 70s inland and
lower 70s near the lake.

As we enjoy the break in the weather Tuesday, another shortwave
trough is forecast to be traversing the US-Canada border and
eventually pivot into the northern Great Lakes Tuesday night.
Associated with this wave will be a cold front which is progged
to move through northern IL and northwest IN on Wednesday. While
this front does look to bring some scattered showers and
thunderstorms with it, the limited moisture return ahead of it
and the northward trajectory of the shortwave may limit storm
coverage locally. For now saw no reason to adjust the offered
POPs from the NBM which keeps 30-50% chances across our area,
but suspect the better storm coverage may be further north than
shown in the current forecast. That said, if sufficient
instability can be realized Wednesday afternoon the 30-35 kts of
shear may allow for a few storms to become strong to severe.

In the wake of the front Wednesday night, guidance has trended
towards a drier end to the work week as modest mid-level height
rises move overhead. However, the signal remains for a pattern
shift towards more west-southwest flow aloft next weekend and
beyond which looks to support the return of more shower and
thunderstorm chances but details at this range remain fluid.
Regardless, the pattern shift also looks to allow temperatures
to warm towards more typical summer conditions this weekend into
next week as well.

Yack

&&

.AVIATION /00Z TAFS THROUGH 06Z TUESDAY/...
Issued at 624 PM CDT Sun Jun 21 2026

Aviation forecast concerns for the 00Z TAFs:

- Period of rain, occasionally heavy, through this evening with
  associated MVFR and IFR conditions.

- MVFR ceilings may linger Monday morning near Lake Michigan,
  and as late as midday for KGYY right off the lake.

- Breezy east to northeast winds gusting 20-25 kts. Winds become
  breezy north-northeast Monday, gradually diminishing late.

Surface low pressure was analyzed just north of KSTL early this
evening, with a warm front extending southeast across southern
IL and into southwest IN. This low will track slowly east into
central IN after midnight, allowing surface winds across the
terminals to gradually back from easterly to north-northeasterly
through early Monday morning. Within the tighter surface
pressure gradient north of the low, surface winds will be
breezy/gusty at times in the 20-25 kt range (potentially a
little stronger at KGYY this evening). Breezy north-northeast
conditions will persist during the day Monday, but will
gradually ease with time as weak surface high pressure spreads
into the Mississippi Valley.

In addition, an area of moderate to at times heavy rain was
occurring north of the surface low track, and this will persist
across the terminals through the evening hours before gradually
tapering off after midnight. Ceilings have been a little slower
to come down than earlier expected, though starting to see some
more solid MVFR bases on the KORD/KMDW 1-minute ASOS output and
this should only continue to deteriorate with continued rain and
saturation of the low-levels. A period of IFR ceilings (and an
occasional visibility) is expected for the Chicago terminals
this evening, with otherwise MVFR conditions expected into early
Monday (though visibilities will improve as rain ends late
tonight). KRFD, farther on the northern periphery of the rain
shield, is less likely to see IFR conditions and should be
quicker to improve to VFR Monday morning.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement from 10 PM CDT this evening through
     late Monday night for ILZ006-ILZ103.

     Beach Hazards Statement through late Monday night for ILZ104.

IN...Beach Hazards Statement through late Monday night for INZ001-
     INZ002.

LM...Small Craft Advisory until 1 AM CDT Tuesday for the IL
     nearshore waters.

     Small Craft Advisory until 4 AM CDT Tuesday for the IN
     nearshore waters.

&&

$$

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